Investigating the Effect of Light Intensity on the Size of a Plantain Leaf
Length: 1322 words (3.8 double-spaced pages)
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Title: To investigate the effect of light intensity on the size of a
Hypothesis: I predict that the size of the plantain leaves would
increase as the light intensity decreases. Therefore, plantain leaves
found in the shade will have larger surface areas than leaves found in
an open area.
Theory: Sunlight is an essential factor need to complete the process
of photosynthesis. Photosynthesis consists of the following equation:
Carbon dioxide + Water ========> Glucose + Oxygen
Chlorophyll is a substance found in chloroplasts, found in the cells
of leaves. They are used to produce glucose which is used as plant
food and growing materials (e.g. cellulose).A leaf which is exposed to
plenty of light will have sufficient amounts of food and will not need
an excessive amount of chlorophyll. This enables the leaf to have a
small surface area. It is also necessary for leaves in areas of high
light intensity, and thus high temperature, to have small leaves to
reduce the amount of transpiration. The heat will cause water to
evaporate a lot faster.
Leaves in shaded areas will need a large surface area full of
chlorophyll to collect as much sun light as possible; essential for
survival. These leaves will also have no threat of excessive
transpiration because the temperature in the shaded area will be lower
and the humidity probably higher.
Transpiration is the removal (evaporation) of water from a plant
through the stomata in the leaves; this water is removed in a cycle
due to the active uptake from the roots. Transpiration involves
osmosis; which is the diffusion of water from a high concentration to
a lower concentration through a partially permeable membrane, until
both the concentrations are equally saturated.
All these factors i.e. transpiration and photosynthesis, come together
to confirm my hypothesis.
To support my hypothesis further, I did a pilot study in a meadow in
which I studied the population of certain plant species in areas of
different light intensities. I learned that different plants need
different environmental factors to grow well. The leaves in the shade
were larger than those in the open field.
As the light intensity increases, the surface area of the leaves
decreases. This sketch shows a line of best fit; in real life it
probably wouldnÂ’t be so accurate.
Variables: The independent variable in my investigation is the light
intensity in the area we are investigating. The other independent
variables in the experiment have to be kept constant. If they are not
kept constant, then it will not be a fair investigation. The
independent variables I am keeping the same in this investigation are:
1. Temperature Â– change in temperature might affect the rate of
2. Humidity of the field: Â– this will also affect the rate of
transpiration as it will determine the concentration gradient
during osmosis from the stomata.
3. Wind speed Â– this will affect the humidity of an area.
4. Moisture level Â– this is an important factor in photosynthesis.
If there is a variation in the moisture levels then the level of
transpiration will vary, making the investigation unfair.
5. Amount of human and animal activities - this will always have an
effect on the environment and should be prevented as much as
The dependent variable in this investigation is the change in the leaf
* A small quadrat 25cm x 25cm
* Papers Â– lined and graph paper
* Sun light meter
* Humidity and Temperature meter
Skill area O: Obtaining evidence
1. Get together all the apparatus listed above and pick out the site
for your experiment. Prepare a table to record the data collected.
2. Look around and gauge which areas are shaded and in full light.
3. Throw the quadrat somewhere as randomly as possible towards a
shaded area. If there are no plantain leaves in the quadrat,
continue to throw it until you get
4. When you have found a good plantain leaf sample, measure the
light intensity, humidity and temperature over the quadrat.
5. Take a leaf sample from the quadrat and trace it unto some graph
paper. Also record the data collected in step 4 on your results
6. Repeat steps 3 Â– 5 in different light intensities i.e. the shade,
full light and moderately lit areas.
7. Calculate the surface areas of the leaves using the squares on
the graph paper. You will have to estimate the surface area and
round them to the nearest unit. (1.s.f.).
8. Record all your data into the table you previously prepared.
* Make sure you stand clear of the path of the quadrat being thrown.
Avoid throwing it to close to the river too.
* When using the sunlight meter, make sure there are no objects
obstructing the reading i.e. stand back from the meter.
* To make the test fair, throwing the quadrat should be a completely
random process. This can be achieved by closing ones eyes when
throwing the quadrat, or to spin around in a circle to confuse oneself
of bearings or position.
* To also make the test fair, make sure the same person carries out
each measurement i.e. the same person measures the light intensity as
their opinion will always be the same.
Light Intensity (klux)
Area where leaf was found
Surface Area (cm2)
Skill area A: Analysing evidence and drawing conclusions
On my graph I can see a definite pattern. In each of the 3 columns,
showing the locations of the leaf samples, the points on the scatter
graph all moved down in descending order as the light intensity
increased. The points in the first column, Â‘Under the treeÂ’; rose to
the highest point i.e. highest surface area which was 45 cm2. The
beginning of each Â‘locationalÂ’ column always has the highest surface
area point. This suggests that the specified areas will be overlapping
each other; they are not clean cut at all.
The pattern on my graph fits my hypothesis exactly. It shows that as
the light intensity increases, the size of the leaves decreases. The
temperature on the day we did our experiment was not very high, but I
could still see the clear difference between the leaf sizes.
Transpiration was not encouraged by the low temperature, and also the
humidity was very high which would have reduced the osmosis gradient
and slowed down transpiration.
Skill area E: Evaluation
I think my investigation was not as great as I would have liked. I
admit that my lack of enthusiasm at the beginning might have
contributed to it, but also because the working conditions we had were
not great. As you can see from the table of results, it was a very
humid environment, it was cold and cloudy; all these were not exactly
the right conditions to get good results. However, we did get adequate
results that substantiated my hypothesis.
My investigation could have been improved in many ways, most of all if
we had the optimal weather conditions we need. To have made it a fair
test, and made sure all the other independent variables remained
constant; I could have used a controlled environment like a
greenhouse. With a larger area to investigate with fuller trees and
more working time; I could have collected more samples. I would have
used this to make the graph my informative and specific. Also, testing
other plant species other than the plantain would have been useful
e.g. ribwort, dandelions.
To further my experiment I actually investigated another plant species
growing with the plantain leaves, dandelions. I carried out the method
stated above to collect my dandelion samples and recorded my results
in the table below.
Light Intensity (klux)
Surface Area of leaves (cm2)
This table and graph also confirm that my initial hypothesis was